This invention relates generally to vehicle rearview mirror systems and, more particularly, to electro-optic mirror assemblies, such as electrochromic rearview mirror assemblies for a vehicle.
Electrochromic rearview mirror assemblies include an electrochromic reflective element made up of a reflecting surface and an electrochromic cell positioned between the driver and the reflecting surface. The electrochromic cell responds to a direct current (DC) voltage applied across a pair of terminals by varying the light transmittance through the cell. In this manner, the reflectance level of the reflective element can be varied by varying the DC voltage applied to the electrochromic cell. The electrochromic cell has characteristics which make control of the reflectance level of the reflective element difficult. The electrochromic cell operates at a relatively low voltage, typically which may not exceed approximately 3 volts DC, more typically not more than about 1.5 volts DC, for more than a brief period of time or else useful life of the reflective element is compromised. Furthermore, the amount of drive current necessary to color or bleach the cell varies both with the temperature of the cell and the amount of change in light transmittance undertaken. Therefore, optimum control of the electrochromic cell requires more than merely applying a DC voltage corresponding to the desired reflectance level.
One approach to controlling the reflectance level of an electrochromic cell is disclosed in commonly assigned U.S. Pat. No. 5,715,093 issued to the present inventor and Niall R. Lynam, entitled AUTOMATIC REARVIEW MIRROR SYSTEM WITH AUTOMATIC HEADLIGHT ACTIVATION. In this co-pending application, the electrochromic cell is driven by an analog feedback system which translates a desired reflectance level, produced by an analog circuit, to a signal applied to the electrochromic cell which drives the cell to the desired reflectance level. While such drive system is effective, it requires the use of analog components. Such analog components would be redundant in a digital electrochromic mirror system and, therefore, would unnecessarily add to the cost of the system. However, substitution of digital components for the previously used analog components is not a straightforward matter. Digital components typically operate between discrete output states which may include binary devices, such as transistors, switches, and the like, which exhibit a low and a high state, and tristate devices, such as types of microprocessors which exhibit a neutral, a low, and a high state. Such components are useful in processing data but are not readily adapted to controlling the reflectance level of an electrochromic rearview mirror. In particular, a typical electrochromic mirror utilized as an interior mirror of a vehicle may have a surface area in the range of 90 cm.sup.2 to 150 cm.sup.2 and typically in the range of 110 cm.sup.2 to 130 cm.sup.2. A steady state current draw, after color transitions have settled, is typically in the range of between approximately 60 milliamperes and 180 milliamperes with a range of 80 milliamperes to 150 milliamperes being typical. Exterior rearview mirrors can be even larger with a surface area of approximately 350 cm.sup.2, and greater, and a commensurate increase in current density.